This invention relates to a process for transferring metal ions from an aqueous liquid A to a non-aqueous liquid B and from non-aqueous liquid B to an aqueous liquid Z, B taking up the metal ions from A in a first transfer zone and carrying them to a second transfer zone in which it releases the metal ions to Z. Liquid B is immiscible or substantially immiscible with the liquids A and Z, contains at least one complexing agent for the metal ions and is a solvent for the metal ion complexes. Liquid B is continuously circulated through the transfer zones, liquid A being kept separate from the circulated liquid B by one or more microporous hydrophobic membranes of which the pores contain some of the total quantity of liquid B.
Processes in which metal ions have to be removed from an aqueous solution are used, for example, in the dressing of ores. In this case, it is frequently desirable selectively to transfer ions of a certain type from an aqueous liquid and to recover the metal in pure form therefrom. Another field of application for processes of this type is effluent treatment where metals have to be removed substantially completely from aqueous liquids.
Processes for transferring metal ions from one aqueous solution to another with complexing in between are already known. Thus, German Offenlegungsschrift No. 29 10 793 describes a process in which an organic liquid containing a complexing agent is accommodated in the pores of a membrane. One surface of the membrane is in contact with the aqueous liquid which releases metal ions, while the other surface is in contact with the aqueous liquid taking up the metal ions. Accordingly, the metal ions are transferred by complexing on one surface of the membrane, diffusion of the complexes through the pore system, and release of the ions at the other surface of the membrane. The disadvantage of processes of this type is that the complexing agent can escape from the pore system and penetrate into the aqueous liquids. This can be produced by a certain solubility of the complexing agent in water or by pressure variations or by turbulence in the aqueous liquids at the surfaces of the membrane. These losses of complexing agent result initially in a slowing down of the process, i.e. in a reduction in the flow of metal ions, and ultimately in the stoppage of the process. Before they can be reused, the membranes have to be cleaned and refilled.
In other processes, the problem discussed above is overcome by not using a complexing agent which remains stationary in the pores of the membrane. Instead, the organic liquid containing the complexing agent is transferred from a first transfer zone to a second transfer zone. In the first transfer zone, the metal ions are taken up from one aqueous liquid by the complexing agent and, in the second transfer zone, are released to another aqueous liquid. Processes of this type, which operate on the principle of extraction and segregation of liquid systems by separation, are described in Chemical Engineering 1970, pages 82 et seq. and in U.S. Pat. No. 3,939,203. These processes also have disadvantages. Thus, undesirable impurities can pass from the aqueous liquid releasing metal ions into the organic liquid while the two liquids are being intensively mixed. These impurities then have to be removed by purifying operations before the pure metals are recovered.
U.S. Pat. No. 3,957,504 describes a process in which an organic phase, which contains a complexing agent and which may be circulated, takes up metal ions from an aqueous liquid and releases them to another aqueous liquid. In this process, the two aqueous liquids are kept separate from the organic phase by membranes. This process is attended by the disadvantage that the flow of metal ions, i.e. the quantity of metal ions transferred per unit of time from one aqueous liquid to the other, is limited due to the use of membranes both on the so-called "feed" side and on the "strip" side (on the "feed" side, the ions are transferred from the aqueous phase to the organic phase; on the "strip" side, they are transferred from the organic phase to the aqueous phase).
Accordingly, the object of developing the process according to the invention was to provide a process which does not have any of the disadvantages mentioned above and which, in particular, represents a more economical alternative to known processes.